Phelps will race in eight events at the Pan Pacific Championships, which begin on Thursday in Australia. It will be Phelps' first international meet since coming out of his 20-month retirement and will be key to the 29-year-old's preparations for next year's World Championships in Russia and the 2016 Rio Olympics.

Phelps qualified for the US team at trials in California earlier this month, finishing second in the 100 metre butterfly and 200m individual medley. He also finished seventh in the 100m freestyle and sixth in the 100m backstroke.

Asked about his form, Phelps said: "I have been working on the basics that hopefully make a big improvement with my freestyle stroke.

"At the nationals, I missed a couple of walls, I jammed a couple of walls. Those were the big things that I guess I forgot how to do. With some extra practice, I hope it's going to pay off."